Many mobile devices, such as mobile phones and tablet computing devices, include cameras that may be operated by a user to capture still and/or video images. Because the mobile devices are typically designed to be relatively small, it can be important to design the cameras or imaging systems to be as thin as possible in order to maintain a low-profile mobile device. In various conventional devices, the thickness of the mobile device is maintained as small as possible by turning the imaging sensor on its side and using reflective devices to bend the rays to the sensor. However, this particular solution is not very robust because the width of the imaging sensor and the pixel pitch may determine the highest resolution of the camera.
In other conventional devices, the thickness of the mobile device is maintained as small as possible by shortening the focal length of the imaging system. However, various problems can arise when the focal length of the system is designed to be as short as possible. For example, because the focal length and the field of view of the system are inversely related, shortening the focal length may increase the field of view to such an extent that the image may cross over from a natural image to an unnatural wide field of view image, e.g., at fields of view of about 60 degrees or greater. Furthermore, lens roll-off, e.g., the loss of light or brightness near the edges of an image as compared with the center of an image, may increase as focal length decreases. Thus, as the focal length of imaging systems in mobile devices is reduced, image quality may degrade due to undesirable lens roll-off.
In addition, the modulation transfer function (MTF) of imaging systems in various mobile devices may produce lower resolution near the edges of the image as compared to the center of the image, which may be referred to as MTF roll-off. Because MTF roll-off may also be inversely related to focal length, a reduced focal length in mobile devices may reduce the resolution at the edges of the image. Moreover, shorter focal lengths may increase the angle of incidence of the lens' chief ray, which can also generate undesirable artifacts, including increased light and electrical crosstalk between pixels and a lower sensor MTF performance. Accordingly, it can be advantageous to reduce the thickness of imaging systems in mobile devices, while maintaining or improving image quality.